In a plurality of practical applications of measurement or sensor technology it is useful to precisely determine a small variable signal portion, since a measurement quantity or an environmental influence to be determined causes only a small change in a physical property of the sensor, or the component, said physical property being accessible to measurement. The measurement quantity detected by an evaluation device at the component or the sensor may also be changed in an undesired manner by further external environmental influences, so that the measurement value will be distorted. This may be caused, for example, by variation in ambient pressure and temperature, as long as these are not the physical measurement quantities to be detected by the component or sensor.
The smaller a desired change in the measurement quantity of the component which is caused by the physical measurement quantity to be determined, the more severe the impact that the above-mentioned additional changes or interferences of this measurement quantity may have on the measurement result. This may even cause the measurement result to be distorted to such an extent that it is no longer meaningful.
A bolometer and the evaluation device or evaluation electronics used for reading out a bolometer may serve as examples. A bolometer serves to measure temperature and/or to measure intensity of radiation in that electromagnetic waves are absorbed within the bolometer. As a result, the temperature of the bolometer increases, the temperature change triggered by the incident heat radiation being very small. Sometimes, temperature differences of less than 1 mK may be resolved. The temperature is determined by a temperature-dependent, electrically functional component and is converted to an electrical signal. As an example of a simple bolometer, an electrical conductor mounted within a vacuum may be mentioned which undergoes a change in resistance as the temperature changes, which change in resistance in turn may be determined by detecting a current which flows through the wire at a constant voltage. In this example as well as in alternative evaluation devices for reading out the bolometer, electrical power dissipation arises within the read-out bolometer itself. Said power dissipation varies depending on the operating state of the bolometer and is caused by the component used for the readout itself.
What is problematic is that in the bolometers used as examples, the warming-up caused by this power dissipation itself cannot be distinguished from that caused by the electromagnetic radiation (infrared radiation) to be detected. Since with bolometers, the temperature change caused by the power dissipation is typically clearly larger than that of the signal to be measured, i.e. than that caused by the electromagnetic radiation absorbed, countermeasures should be taken with the objective of obtaining a meaningful readout. Compensation for this effect could be achieved, for example, by periodic recalibration as is achieved, for example, by using a shutter in infrared cameras. In this context, the camera is shielded off from the radiation influences by means of the shutter, so that in this shielded-off state of the sensor element, said sensor element may be recalibrated. However, during this time the camera is blind and cannot take any pictures. A further possibility would consist in specifying the operating parameters of the sensor or component in great detail while taking into account the power dissipation introduced by the readout. Before the measurement values determined are deemed meaningful, one could wait until a stationary state is achieved, i.e. until the long-term time average of the operating parameters is reached. This stationary state could also be reached by external control involving a large amount of effort. For a bolometer, this would mean, for example, to keep the temperature of the substrate constant, which may be achieved, for example, by means of a thermoelectric cooling module (a Peltier element) or the like. The control associated therewith entails a large amount of effort. Alternatively, the temperature of the substrate could be measured, and the measurement value could be corrected using many calibration points which have been detected for different temperatures. This, too, entails a very large amount of effort, and, additionally, does not result in full compensation being achieved, since the individual structural elements, which are manufactured micromechanically in most cases, exhibit considerable variations with regard to their parameters.
Deviation between individual sensors may further lead to secondary effects, since, for example with the above-described resistance bolometers, a differing resistance or a differing temperature coefficient directly influences not only the output signal, but also the level of the power dissipation generated by the evaluation device within the sensor, and thus, in turn, indirectly influences the output signal.
Even though the above as well as the following discussions mainly relate to electronic evaluation devices, similar problems of the feedback of evaluation devices onto the readout result also arise with mechanical systems, for example. For example, if the amplitude of oscillation of an oscillation system is to be determined mechanically, the mechanical coupling of the evaluation device the system to be determined will inevitably result in power being supplied to or withdrawn from the system, which in turn will distort the readout result.
There is thus a need to provide evaluation devices which enable more reliable determination of a measurement value at a component.